title: Cobinamide production of hydrogen in a homogeneous aqueous photochemical system, and assembly and photoreduction in a (βα)8 protein.
abstract: Components of a protein-integrated, earth-abundant metal macrocycle catalyst, with the purpose of H2 production from aqueous protons under green conditions, are characterized. The cobalt-corrin complex, cobinamide, is demonstrated to produce H2 (4.4 ± 1.8 × 10(-3) turnover number per hour) in a homogeneous, photosensitizer/sacrificial electron donor system in pure water at neutral pH. Turnover is proposed to be limited by the relatively low population of the gateway cobalt(III) hydride species. A heterolytic mechanism for H2 production from the cobalt(II) hydride is proposed. Two essential requirements for assembly of a functional protein-catalyst complex are demonstrated for interaction of cobinamide with the (βα)8 TIM barrel protein, EutB, from the adenosylcobalamin-dependent ethanolamine ammonia lyase from Salmonella typhimurium: (1) high-affinity equilibrium binding of the cobinamide (dissociation constant 2.1 × 10(-7) M) and (2) in situ photoreduction of the cobinamide-protein complex to the Co(I) state. Molecular modeling of the cobinamide-EutB interaction shows that these features arise from specific hydrogen-bond and apolar interactions of the protein with the alkylamide substituents and the ring of the corrin, and accessibility of the binding site to the solution. The results establish cobinamide-EutB as a platform for design and engineering of a robust H2 production metallocatalyst that operates under green conditions and uses the advantages of the protein as a tunable medium and material support.
authors.affiliation: Department of Physics, N201 Mathematics and Science Center, Emory University, 400 Dowman Drive, Atlanta, GA 30322-2430, USA.
authors.forename: Wesley D
authors.initials: WD
authors.lastname: Robertson
authors.forename: Adonis M
authors.initials: AM
authors.lastname: Bovell
authors.forename: Kurt
authors.initials: K
authors.lastname: Warncke
mesh_terms: D001665:Binding Sites; D003035:Cobalt / Q000737:chemistry / Q000378:metabolism; D003038:Cobamides / Q000737:chemistry / Q000378:metabolism*; D004981:Ethanolamine Ammonia-Lyase / Q000737:chemistry* / Q000378:metabolism*; D006859:Hydrogen / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D012486:Salmonella typhimurium / Q000378:metabolism; D014867:Water / Q000737:chemistry / Q000378:metabolism
chemical_list: D003038:Cobamides; D009942:Organometallic Compounds; D014867:Water; C017142:cobinamide; D003035:Cobalt; D006859:Hydrogen; D004981:Ethanolamine Ammonia-Lyase